Method for repair of coatings on carbon - carbon composites

ABSTRACT

A method of repairing damage to an oxidation-resistant layer on a carbon-carbon composite wherein a composition consisting essentially of a polymeric thermosetting polysilazane and a ceramic powder is applied to the damaged area, cured, and fired.

This invention was made under United States Government Contract No.F33657-86-C-2124 and the United States Government has a non-exclusive,non-transferrable, irrevocable, paid-up license to practice or havepracticed for on behalf of the United States, this invention throughoutthe world.

CROSS REFERENCE TO RELATED APPLICATION

The instant application is a continuation-in-part of U.S. patentapplication Ser. No. 07/764,793 filed Sep. 24, 1991, now U.S. Pat. No.5,294,425.

BACKGROUND OF THE INVENTION

The present invention relates to a method of repairing damage to anoxidation-resistant coating on a carbon-carbon composite. The inventionis based, in part, on utilization of polymeric ceramic precursorsdisclosed in the parent application identified above, whose entirespecification and claims are specifically incorporated herein byreference.

As is well known, carbon-carbon composites are formed of carbon fibers,typically woven into two-dimensional weaves which are stacked andsurrounded by a dense carbon matrix. Such composites can be flat orshaped into three-dimensional structures and are widely utilized in theaerospace industry performing many vital components, such as in theSpace Shuttle Orbiter Vehicle.

In order to prevent oxidation of the carbon in such carbon-carboncomposites, it is common and necessary to apply a layer of anoxidation-resistant ceramic coating onto the surface of the composites.At present ceramic coatings composed primarily of silicon carbide orsilicon nitride are used to protect carbon-carbon composites fromoxidation. These coatings are applied by known and conventionaltechniques such as pack cementation, chemical vapor deposition, or acombination of these procedures. Although these methods produce coatingswhich impart a useful lifetime to the carbon structures, they requireextreme conditions and specialized equipment for application. Structureswith damage to the oxidation protection coating that result in exposureof the carbon-carbon substrate must be removed from the vehicle andshipped offsite for repair. Techniques which permit repair coatings inthe field are or on the shop floor are not available and the result iscostly in terms of time and manufacturing procedures, to repair damagedcarbon-carbon structures and in the time required to make the repair.

SUMMARY OF THE INVENTION

The present invention overcomes the problems of the prior art to providea method of repairing damage to the oxidation-resistant coatings oncarbon-carbon composites, which repair can be accomplished in the fieldor on the shop floor and drastically reduce the time and cost associatedwith the present repair procedures.

Briefly stated, the present invention comprises a method of repairingdamage to an oxidation-resistant layer on a carbon-carbon compositecomprising applying to said damage a coating of a composition consistingessentially of a polymeric thermosetting polysilazane and a ceramicpowder, curing the composition, and firing the composition coatedcarbon-carbon composite at a temperature and for a time sufficient toconvert said polysilazane to a ceramic.

DETAILED DESCRIPTION

As noted above, the method of making the individual carbon-carboncomposites themselves are well known and any conventional method can beutilized to form the same. In like manner, the oxidation-resistantceramic coatings and the method of preparing them and applying them tothe carbon-carbon composite are also well known. The instant inventionis applicable to the method of repairing damage to theoxidation-resistant coating after the carbon-carbon composite has beenput into use on an aerospace craft such as the Space Shuttle OrbiterVehicle.

The critical part of the instant invention is a composition which can beutilized to repair the damage, which may be a crack, chip, hole,scratch, or any other damage.

The composition comprises a polymeric thermosetting polysilazane and aceramic powder. As to the polymeric thermosetting polysilazane, it ispreferably a perhydropolysilazane which can be thermally transformedinto silicon nitride. Other specific examples are N-tert-butylhydridopolysilazane and N-n-butyl hydridopolysilazane. The method offorming these preceramic precursor polymers is not a part of thisinvention and with respect to the polysilazanes is disclosed in theparent application noted above.

As to the ceramic powder utilized, it is preferred to use siliconcarbide, silicon nitride, and the like, preferably one having a micronsize no greater than about 10 microns and most preferably a sub-micronaverage particle size; less than about 0.5 micron. It is also possibleto use ceramic whiskers and mixtures of the same with the ceramicpowders. As used herein, the term "ceramic powder" is meant to includesuch whiskers. It is preferred that the purity of the powders andwhiskers be 99% or greater.

As to proportions, it is preferred to use for each 100 wt. % of thecomposition about 50 to 80 wt. % of the ceramic powder and preferably 70wt. %. It is best to load the polymer with as much powder as possiblewhile maintaining a paste-like consistency.

The repair composition is formed by simply thoroughly mixing thecomponents described above to form a substantially homogeneous mixtureusing conventional inert atmosphere/vacuum manifold or argon-filleddrybox procedures. As is known, it is necessary to handle the uncuredpreceramic polymers using the known anaerobic and anhydrous conditionsin combination with the just-described procedures.

To repair the damage to the individual carbon-carbon composites, thecomposition described above is applied to the damaged area by spraying,daubing, brushing, or any like procedure. The composites are then heatedunder applied pressure; i.e. about one atmosphere. No autoclave isrequired to cure the polymer. It is preferred to hold the assembly in avacuum bag during curing and it is preferred to effect curing at about200° C. The use of the vacuum bag gives one atmosphere pressure. Curingcan be effected in about six hours, but the optimum temperature and timefor any given composition can be determined by routine experimentation.

After cure, the structure containing the cured "green patch" is removedfrom the vacuum bag and fired. The firing is at a temperature and for atime sufficient to convert the polymeric ceramic precursor, used as partof the composition, to a ceramic. The optimum temperature and time foreach adhesive will, thus, vary, but can be determined by routineexperimentation. For perhydropolysilazane, for example, a temperature ofabout 1450° C. for about 30 minutes is suitable. It is preferred tocarry out the firing under a flowing inert gas, such as nitrogen, andany conventional furnace can be used.

Additional coatings of pure polymer and polymer/powder mixtures areapplied using the same procedure described above to fill in any smallcracks that may result from the shrinkage occurring during thepolymer-to-ceramic conversion process. Typically, no greater than sixtotal coatings are necessary.

The invention will be further described in connection with the followingexample which is set forth for purposes of further illustration only.

EXAMPLE

Four coated RCC-3 specimens (2.78 inch diameter) were received fromNASA-Johnson Space Center (NASA-JSC). A circular damage pattern was madein the oxidation-resistant coating by machining through the ceramiccoating in the center of each disk. The damage pattern had an overallouter diameter of 0.5 inch, while the overall diameter of the exposedsurface of the carbon-carbon composite was 0.25 inch in diameter. Thedamage pattern was machined completely through the oxidation-resistantcoating. Before coating repair was initiated, the machined surfaces wereroughed up with sandpaper to promote mechanical adhesion between thecoating and the substrate. A thin layer of pure polymerperhydropolysilazane (PHPS) was applied with a small paint brush to theroughed up surface, followed by polymer loaded (70%) by weight withsubmicron α-Si₃ N₄ powder (H. C. Stark, LC-12), which was applied with asmall spatula.

The repaired specimens were cured in a vacuum bag under ambient appliedpressure at 180° C. for 6 hours.

After curing was complete, these "green patch" specimens were removedfrom the vacuum bag and fired to 1400° C. under flowing nitrogen. Asecond coating of powder-loaded polymer was applied, followed by threecoatings of pure polymer (no powder loading), using the curing andfiring procedure described above for each, for a total of five coatingcycles. After the fifth coating cycle, the specimens were fired to 1700°C. under nitrogen.

While, for purposes of experimentation, the repair was used only againstthe circular damage pattern, the procedure can be used against allmanner of scratches, chips, dings, cuts, holes and the like. Inaddition, lower firing temperatures are likely to provide sufficientprotection, the 1700° C. firing temperature used in this specificexample was chosen only because of the anticipated test conditions.Also, the nature of the powder additive is not critical, as long as itis a refractory material, such as silicon nitride, silicon carbide, andthe like. Also, other means of maintaining an inert atmosphere andregister of the repair material against the repair site while heat isapplied are capable of producing adequate green patches. It is notnecessary that the powder-loaded polymer be applied with a spatula orthat subsequent coatings of PHPS be applied with a brush. Other methods,such as spray-gun, doctor blade, injector gun, and the like are fullycompatible with this repair method.

The samples prepared as noted above were tested at The Johnson SpaceCenter in the 10MW Atmospheric Reentry Materials & Structures EvaluationFacility (ARMSEF). Specimen surface temperatures were determined using a0.865 μ pyrometer. The experimental matrix is summarized in Table 1.Note that the two specimens which had undergone only five coating cycleswere subjected to two test cycles.

                  TABLE 1                                                         ______________________________________                                                                          Cycle  Accumu-                              Sam- Coating  Test    Temp. Press.                                                                              Duration                                                                             lated                                ple  Cycles   Cycles  (°C.)                                                                        (psf) (seconds)                                                                            Cycle Time                           ______________________________________                                        1    7        1       1593  100   900    900                                  2    5        1       1649  100   450    450                                  2             2       1649  100   450    900                                  3    5        1       1704  100   450    450                                  3             2       1704  100   450    900                                  4    7        1       1840  100   118    118                                  ______________________________________                                    

The test results reveal that no damage to the underlying carbon-carbonsubstrate in tests cycles with temperatures less than 1700° C.

Despite its simplicity, the preceramic polymer repair technique providedexcellent protection to the RCC-3 substrate to temperatures as high as1700° C. under simulated re-entry conditions for short periods. Thepolymer-derived Si₃ N₄ may undergo the transition from passive to activeoxidation at temperatures lower than those found for Si₃ N₄ processedthrough other techniques; however, improved processing to yieldstoichiometric and crystalline Si₃ N₄ has the potential to raisesubstantially the active oxidation threshold.

While the invention has been described in connection with a preferredembodiment, it is not intended to limit the scope of the invention tothe particular form set forth, but on the contrary, it is intended tocover such alternatives, modifications, and equivalents as may beincluded within the spirit and scope of the invention as defined by theappended claims.

What is claimed is:
 1. A method of repairing damage to anoxidation-resistant layer on a carbon-carbon composite comprisingapplying to said damage a coating of a composition consistingessentially of a polymeric thermosetting polysilazane and a ceramicpowder, curing the composition, and firing the composition-coatedcarbon-carbon composite at a temperature and for a time sufficient toconvert said polysilazane to a ceramic.
 2. The method of claim 1 whereinthe damage is first primed with said polymeric thermosettingpolysilazane and said composition then placed thereover.
 3. The methodof claim 1 wherein said polysilazane is perhydropolysilazane and saidceramic powder is silicon nitride, said silicon nitride comprising up to80% by weight of said composition.
 4. The method of claim 2 wherein saidpolysilazane is perhydropolysilazane and said ceramic powder is siliconnitride, said silicon nitride comprising up to 80% by weight of saidcomposition.
 5. The method of claim 3 wherein curing is effected underone atmosphere applied pressure on the damaged portion of thecarbon-carbon composite.
 6. The method of claim 4 wherein curing iseffected under one atmosphere applied pressure on the damaged portion ofthe carbon-carbon composite.
 7. The method of claim 5 wherein at leastone additional coating of said composition is applied over said firedcoating and said at least one additional coating is cured and fired inthe same manner as said first coating.
 8. The method of claim 6 whereinat least one additional coating of said composition is applied over saidfired coating and said at least one additional coating is cured andfired in the same manner as said first coating.